Acid resistant silver-palladium-gold alloys



Aug, U, 1955 J. SCHATZ 2,716,605

' ACID RESISTANT SILVER-PALLADIUM-GOLD ALLOYS Filed Dec. 15, 1952 W/P \iig INVENTOR ATTORNEYS ACID RESISTANT SILVER-PALLADIUM-GOLD ALLOYS llohannes Sclratz, Frankfurt am Main, Germany, assignor to Dentsche Goldund Silber-Scheideanstalt vormals Roessler, Frankfurt am Main, Germany Application December 15, 1952, Serial No. 326,038 Claims priority, application Germany December 22, 1951 2 Claims. (Cl. 75--173) The present invention relates to silver-palladium-gold alloys which are highly resistant to corrosion by acids and more particularly to such alloys having a relatively low gold content, which are especially adaptable for chemical apparatus and parts thereof which must resist the action of strong or concentrated acids and, if necessary, at elevated temperatures.

Previously noble metals, or alloys with a high content of noble metals, especially gold, were employed when strong corrosive conditions were to be reckoned with. For example, alloys containing about 70% gold and 30% silver have been proposed for such purposes which, because of their high gold content, do possess substantial resistance to corrosion by acids. On the other hand, such alloys have other properties which are disadvantageous which sometimes limit or even exclude their use for certain purposes.

It has also been proposed to employ alloys which besides palladium and silver, contain at least 20% and generally 20% to 65% gold. These alloys were developed upon the basis of the discovery that the acid resistance of the gold-silver-palladium system alloys show a marked increase in a limited range of concentrations and that it did not increase uniformly with increases in gold content.

in accordance with the present invention, it has now been unexpectedly discovered that the addition of a relatively non-noble metal, chromium, to silver-gold-palladium alloys displaces this range of high acid resistance towards the lower gold concentrations so that alloys with 30 to 70% of silver, 15-50% of palladium and only 5 to 20% of gold which also contain 0.05% to 2%, preferably 0.2% to 0.9% of chromium can be employed in the production of chemical apparatus or parts thereof which are subject to the action of strong or concentrated acids especially mineral acids even at elevated temperatures. It could in no way be foreseen that the addition of a relatively small quantity of chromium to silver-palladium-gold alloys would cause such alloys of lower gold content to have the same acid resistance as that of palladium-silver-gold alloys containing substantially greater quantities of gold as Well as that of the previously employed gold-silver alloys containing about 70% of gold.

The acid resistance of the alloys according to the invention is indicated in the following table which records the loss in thickness per year of a number of alloys according to the invention which were subjected to the corrosive vapors of 20% HCl boiling at 110 C. with access to air.

nited States Patent 0 p Patented Aug. 30, 1955 ice Similar results are obtained with corrosion tests in other corroding media and the alloys according to the invention are practically completely stable against cold or hot mineral acids such as hydrochloric acid, sulfuric acid and even against oxidizing acids and mixtures thereof. When test samples were placed in the boiling HCl instead of in the vapors evolved as was the case in the tests recorded above, the losses in thickness amounted to only one sixth to one tenth of those shown in the table.

The most preferred compositions of the alloys according to the invention with reference to corrosion resistance and amount of gold employed are those containing 30-70% Ag, 15-50% Pd, 10-15% An and 0.2-0.9% Cr.

It was found according to the invention that the best corrosion resistant properties of the alloys is achieved with such relatively small amounts of chromium that its presence does not or substantially does not render the structure of the alloys heterogeneous. In other words, it was found that the greatest increase in resistance to acids was achieved in alloys which were of substantially single phase or homogeneous nature and that alloys containing such quantities of chromium that their structure was more than slightly heterogeneous did not possess the desired corrosion resistance.

The chromium containing alloys according to the invention can be employed with advantage in all instances wherein highest acid resistance and optimum mechanical properties are required, as they are not only highly resistant to acids, but also possess favorable mechanical properties and workability. They are especially suitable for the production of chemical apparatus, for example, parts of mixers which operate in contact with acids and valve fittings, acid pumps, injection or distributing nozzles which are contacted with flowing acids. They are also suitable for the construction of larger apparatus, for example, kettles or tubing which are subject to the action of concentrated cold or hot acids. The apparatus in question can be constructed completely of the alloys in question or they can be clad or plated on the surfaces in contact with acids with such alloys. It has been found especially advantageous if the alloys are plated upon a silver base or if they are plated over a silver coating applied to a steel base.

The economic advantages of the alloys according to the invention are substantial as, for example, the cost of an alloy containing 59% Ag, 30% Pd, 10% Au and 1% Cr is only 78% of a gold-silver alloy containing 60% Au and onlyabout 45% of palladium-gold-silver alloy containing a greater quantity of gold.

The accompanying drawings serve to illustrate the eifect of the addition of chromium to the silver-palladiumgold alloys according to the invention upon the corrosion resistance to acids. The corrosion medium employed was the vapor of 20% HCl at 110 C. with access of air In the drawings:

Fig. 1 shows the corrosion resistance of a silverpalladium-gold alloy containing 20% Pd with varying amounts of gold with and without addition of chromium according to the invention;

Fig. 2 shows the corrosion resistance of a silverpalladium-gold alloy containing 30% Pd with varying amounts of gold with and without addition of chromium according to the invention.

I claim:

1. An acid resistant alloy composed of 35 to 70% of silver, 15 to 50% of palladium, 5 to of gold and 0.2 to 0.9% of chromium.

2. An acid resistant alloy composed of to 70% of silver, 15 to of palladium, 10 to 15% of gold and 0.2 to 0.9% of chromium.

References Cited in the file of this patent UNITED STATES PATENTS 1,946,231 Nowack Feb. 6, 1934 1,952,083 Leach Mar. 27, 1934 FOREIGN PATENTS 611,709 Germany Apr. 3, 1935 621,152 Germany Nov. 2, 1935 

1. AN ACID RESISTANT ALLOY COMPOSED OF 35 TO 70% OF SILVER, 15 TO 50% OF PALLADIUM, 5 TO 20% OF GOLD AND 0.2 TO 0.9% OF CHROMIUM. 